Fraud when it comes to body print capture, notably, commonly, fingerprint capture, can, according to one of its aspects, consist of the so-called “false finger” technique: the defrauder uses a false finger (or a false area of the body) reproducing the body print characterizing an individual. This false finger can, for example, consist of an imitation of a complete finger (or of an area of the body) provided with a reproduction of the body print to be detected to characterize a given individual, or simply, concerning the capture of a fingerprint, this false finger can consist of a finger stall, provided externally with a reproduction of the fingerprint of the individual, with which the falsifier clads his own finger.
These false fingers, which can certainly offer the formal appearance of the appropriate body print, do not, however, have all the electrical or chemical characteristics of a living finger. It is therefore known to identify the living character of an area of the body from which a fingerprint is to be taken by performing appropriate electrical measurements when said area of the body is applied to a fingerprint capture device. For example, it can be an electrical resistance measurement as taught in documents FR 2 849 244 and FR 2 849 246, both in the name of the Applicant.
However, it is not possible to exclude the fact that the fraud can take place with the proper body area of the individual that the defrauder would have previously amputated (in particular, cutting off a finger of the individual that the defrauder then applies to the fingerprint capture device). There is therefore a need, to avoid this fraud method, to verify the living nature of the area of the body placed in cooperation with the fingerprint capture device.
However, the existing control methods often prove ill suited to this precise case of fraud because a body area that has been amputated loses the physical characteristics measured by these methods only after a certain time period, sufficiently long to allow a defrauder the time to amputate an area of the body of the individual, then go to the print capture device to use thereon the area of the body amputated.
To overcome these constraining drawbacks, it therefore appears advantageous to address the living character of the body area on which the biometric capture must be performed, and this living character is reflected in the proportion of oxygenated hemoglobin relative to the proportion of non-oxygenated hemoglobin of said body area. Such a solution is of particular interest, because this ratio [oxygenated hemoglobin/non-oxygenated hemoglobin] remains approximately constant as long as the tissues are living, but decreases very rapidly immediately the tissues cease to be living (case of a body area amputated, such as a finger that has been amputated for example) or are traumatized with the blood circulation stopped or significantly reduced (case of a finger or an arm fitted with a tourniquet, for example); typically, the hemoglobin oxygenation/deoxygenation ratio has already strongly decreased from the first minute at the end of which the oxygenation is no longer renewed because the cells do not die instantaneously and finish consuming the oxygen locally present (case of a body area that has been amputated or immediately the heart is stopped for example). However, it is improbable that the defrauder will amputate the body area in the same place where the print capture must be performed and in practice, between the moment when the body area is amputated and its use for the purposes of print capture, a time interval will elapse which, even if it is not very long (for example a few minutes, and a fortiori if it is several hours, even several days), will be sufficient for the oxygenation ratio of the hemoglobin to have decreased to a value very much lower than the living value. From this point of view, such a method proves more efficient than most of those used to date.
Of course, it is already known, in the medical field, to proceed with light transmission measurements at several wavelengths on one position of the body of an individual in order to deduce therefrom, in conjunction with a pulse measurement, the oxygenation rate of the arterial blood (in this case, only the pulsed arterial part in pace with the pulse is extracted to eliminate the influence of the pigments, of the bones and of the dark blood). This quantity is represented by the amount of oxygenated hemoglobin relative to the total hemoglobin present. It is used, for example, to detect drops in oxygenation during sleep apnea. This is a measurement process that is slow, notably because of the long time needed to carry out the measurement of the pulse, but it does lead to accurate information concerning the oxygenation rate of the arterial blood.
However, it is essential to consider that such precise information on the oxygenation rate of the arterial blood, while it can indeed be exploited in the context of a biometric identification of an individual targeted by the invention, would be excessive, because the precision of the information to which it would lead is not necessary to arrive at the biometric information sought; furthermore, the acquisition time for the useful information would be much too long and implementation relatively complex (modulation of the incident light, numerous processing operations to extract the pulsed information and spurious information), such that this known solution cannot be retained to implement a fast and economical process of biometric identification. Also, optical analysis in transmission mode is not suited to the usual format of the biometric sensors currently used.
It is essential to understand that, in the context of the invention, it is enough, to achieve the required result, to have information, however partial (by comparison to the absolute measurement performed in the medical field), that is simply representative of the imperative presence and of the relative equilibrium of oxygenated and non-oxygenated hemoglobin in proportions compatible with the living character in the object (real, falsified or false body area) being examined, without it being necessary or simply useful either to know the precise value of the oxygenation rate as defined in the medical environment, or be limited specifically to the arterial or pulsed blood.